Male songbirds show higher coccidia oocyst burdens than femalesfollowing anthelmintic treatment

In wildlife research, wild-caught vertebrates are often given anti-parasitic drugs when brought into captivity to protect animal health and reduce confounding effects of parasitic infection on research outcomes. However, the impacts of antiparasitic drugs on non-target parasite taxa are understudied, especially regarding host sex. To help address this gap, we investigated the impact of an anthelmintic medication on a protozoan gut parasite by quantifying coccidia oocyst burden in the feces of wild-caught dark-eyed juncos (Junco hyemalis) before and after two oral doses of ivermectin. Prior to treatment, males showed higher oocyst burdens than females. Following treatment, ivermectin-treated males showed larger increases in oocyst burden compared to both control males and ivermectin-treated females, while there was no difference in baseline and post-treatment oocyst burden in treated females. Our results suggest that wild-caught songbirds should be housed separately by sex during ivermectin treatment and that male enclosures should be cleaned at a relatively higher frequency due to excessive oocyst shedding. It is unclear whether increases in oocyst burden among males were attributable to helminth removal or a direct impact of ivermectin on host immune function. Further work investigating sex differences in the impact of antiparasitic drugs on non-target parasite taxa is warranted.